TypeScript类型编程中的extends和infer示例解析

type isOne<T extends number> = T extends 1 ? true : false

type GetFirst<T extends string[]> = T extends [infer FirstChar, ...infer Rest]
  ? `${FirstChar}`
  : never;
type Res = GetFirst<['1', '2', '3']>;

type GetFirst<T extends string[]> = T extends [infer FirstChar, ...infer Rest]
? FirstChar extends string
  ? `${FirstChar}`
  : never
: never;

type GetFirst<T extends string[]> = T extends [infer FirstChar, ...infer Rest]
? `${FirstChar & string}`
: never;

type GetFirst<T extends string[]> = T extends [
infer FirstChar extends string,
...infer Rest
]
? `${FirstChar}`
: never;

type StrToNum<T extends string> = T extends `${infer Num extends number}` ? Num : T
type Res = StrToNum<"1">
// ts 4.7时，返回的结果是type Res = number
// ts 4.8及以上， 返回的结果是type Res = 1

type PromiseValue<T extends Promise<unknown>> = T extends Promise<infer Value> ? Value : never
type Res = PromiseValue<Promise<string>> // string

type MyReturnType<T extends (...args: any) => any> 
    = T extends (...args: any) => infer R 
        ? R 
        : any;
type Res = MyReturnType<() => string> // type Res = string

type MyParameters<T extends (...args: any) => any> 
    = T extends (...args: infer P) => any 
        ? P 
        : never;
type Res = MyParameters<(a: string, b: number) => void>  // type Res = [a: string, b: number]